1. Field of the Invention
This invention relates to the field of testing fluid samples and more specifically to testing hydrocarbon samples using acoustic signals.
2. Background of the Invention
There are pressing needs for inexpensive real time interpretation of hydrocarbon samples collected in bottom hole samplers. Typically, a hydrocarbon sample is collected from an underground reservoir primarily for establishing its pressure-volume-temperature (PVT) and flow assurance properties such as the onset of solids.
PVT information about a hydrocarbon sample can include many different types of information. An important type of information is constant composition expansion (CCE) study, which is sometimes referred to as constant mass expansion (CME). In a typical laboratory conducted CCE study, a sample at reservoir temperature or any secondary temperature is taken to a pressure considerably above reservoir and saturation pressures. The sample is then equilibrated, and the pressure is lowered at constant temperature. As the pressure is lowered, the pressure/volume behavior of the sample is recorded. The sample composition does not change during the exercise. The CCE study typically provides the following data about the sample. In the single phase region, the fluid phase compressibility is established, the saturation pressure (bubble/dew point) is recorded at a lower pressure, and the relative volumes of the various phases are reported at pressures below the saturation pressure. The data generated during a CCE study can be important in that it provides a data set that can be effectively used to tune a compositional Equation of State (EOS), which can improve the complete PVT predictions generated. The collection of CCE data at different temperatures can be even more valuable for tuning purposes.
From a flow assurance standpoint, other important information includes the onset of solids formation in the well string and transport lines. Solids primarily take the form of wax and/or asphaltene particles. Wax particles are high molecular weight paraffinic species that precipitate primarily due to temperature drop and can curtail production operations by agglomeration, sticking to pipe walls, congealing in flow lines, and the like. Asphaltenes tend to have a more complex chemical nature than wax and form primarily due to a disruption in a fine balance of interactions that keep them in suspension/solution in the bulk crude. Asphaltene precipitation is usually preceded by a drop in system pressure that leads to gas release and subsequent disruption of the inter-molecular balance needed to keep them stabilized. Once formed, asphaltenes are typically at least as disruptive to flow operations as wax formation. Consequently, measuring wax and asphaltene formation conditions can be an important step to mitigating their flow reduction tendencies.
These PVT and flow assurance properties are typically measured in the laboratory. Drawbacks to the typical laboratory measurements include the time delay involved in laboratory testing and the costs involved with such a delay. For instance, such measurements may not take place for weeks or months after the samples have been collected. Costs for storing and transporting the samples for the laboratory testing can be significant. In addition, the expense of collecting samples can be significant as well for most exploratory environments such as offshore or remote locations. In such environments, only a single opportunity may be available for collecting a sample. Due to such costs and time delay, there is a strong interest in knowing some fundamental PVT and flow assurance characteristics of a freshly captured sample in real time. Other drawbacks include not knowing the quality of a sample or whether a sample was even collected until the expense and delay of laboratory testing has been conducted. The conventional methods of saturation pressure determination in the laboratory include a plot of pressure as a function of sample volume change, with the pressure at which a sharp change in the compressibility occurs defining the saturation pressure. Drawbacks to such conventional methods include the system pressure typically having to be dropped significantly below the saturation pressure in order to define a clear transition point. For some systems, a significant pressure drop below the saturation pressure may result in asphaltene precipitation taking place, which is very difficult to reverse.
Consequently, there is a need for real time testing of hydrocarbon well samples. Other needs include quickly determining whether a sample was collected and its quality. Further needs include a quicker and more cost efficient way to determine PVT information and flow assurance properties of hydrocarbon well samples. In addition, needs include a non-intrusive and non-destructive way to quickly test bottomhole samples.